As an example of the application of a balun consider a high power transistor amplifier. Where output power requirements exceed that which can be delivered by a single transistor, a plurality of transistors may be coupled to share the power demand. In the base station of a cellular radio network, for example, operating in the high frequency region of 900 MHz, an output power requirement at the antenna feed of 1 kwatt would be typical, delivered by ten 100 watt amplifier modules. Presently the upper limit power capability of radiofrequency (r.f.) power transistors is in the region of 60 watt, hence to provide such a 100 watt module coupled transistors must be used.
A convenient and useful method of coupling two transistors is the technique known in the art as "push-pull". Here the drive is shared between a transistor driving current through the load in one direction and a second transistor driving current through the load in the opposite direction. The arrangement is advantageous in terms of impedance matching, presenting a higher output impedance than the very low impedance of a single transistor or the yet lower impedance of parallel transistors, the higher output impedance being closer to the typical target impedance of 50, but unfortunately is inherently a balanced system, that is it produces an output signal which is symmetrical with respect to the common ground potential of the coupled transistors, whereas typically a single-ended (that is ground referenced) output signal is required. A solution to this problem is to provide a transformer between the output of the push-pull pair and the load. By virtue of its isolating properties, a transformer is able to couple the balanced output to the single-ended load. Such a transformer is that referred to in the art as a balun.
In the case of push-pull arrangement, a similar problem exists in providing the complementary balanced drive signals required by the push-pull pair from a single-ended input signal to the amplifier. A balun may be used to convert such an unbalanced input to the balanced drive signals.
Many forms of balun, including transmission line forms, are known in the art. Examples have been described by C. L. Ruthroff in his paper "Some Broad-Band Transformers" (Proceedings of the Institute of Radio Engineers, 1959), pages 1337-1342.
In high frequency r.f. amplifier applications a coaxial cable transmission line balun is typically used, for example a quarter wavelength length of cable having its outer conductor grounded at the single-ended side. An applied single-ended input will then generate a balanced output between the cable conductors at the remote end of the cable; equally a balanced signal applied to the non-grounded side will produce a single-ended output. The incorporation of such baluns in r.f. push-pull amplifier circuits will be known to those skilled in the art.
Although providing good performance, there are a number of drawbacks associated with coaxial cable baluns. The baluns are made by cutting coaxial cable to length and at high frequency even small inaccuracies in length affect performance, also the nature of the cable component is not susceptible to mechanized placement and soldering. Hence the manufacturing tolerances associated with manual assembly create performance variations between one amplifier and another. In an attempt to overcome this problem baluns of printed form, which give good repeatability, have been tried. Printed circuit forms of baluns are known (see for example U.S. Pat. No. 4,193,048 issued to Nyhus). Unfortunately, a conventional construction printed flat on a circuit board cannot be used in high power applications due to the proximity of the large metal heatsinks associated with the amplifying transistors. Inevitably, one side of the circuit board is closer to the metal than the other causing an unacceptable imbalance in parasitic capacitances applied to the balun. To date, only simple printed forms have been used on separate boards mounted normal to the amplifier circuit board to give parasitic symmetry. Such boards are mechanically vulnerable and although repeatable in themselves, cannot be automatically assembled.
The present invention, by contrast, provides a balun construction, useful in high power applications which may be printed flat.